This application is a 371 application of International Application No. PCT/JP97/04633 filed Dec. 16, 1997.
The present invention relates to liquid crystalline compounds and liquid crystal compositions. More specifically, the invention relates to novel liquid crystalline compounds simultaneously having butylene group or propylenoxy group, and 2,3-difluorophenyl group in the compounds; to liquid crystal compositions comprising the compound; and further to liquid crystal display devices fabricated by using the liquid crystal composition.
Display devices produced by employing optical anisotropy and dielectric anisotropy which are characteristics of liquid crystalline compounds (the term xe2x80x9cliquid crystalline compoundsxe2x80x9d is used in this specification as a general term for the compounds which exhibit a liquid crystal phase and for the compounds which do not exhibit a liquid crystal phase but are useful as component of liquid crystal compositions) have widely been utilized for tabletop calculators, word processors, and TV sets including watches, and the demand for the devices are rising year after year.
Liquid crystal phase is broadly classified into nematic phase, smectic phase, and cholesteric phase. Among them, nematic phase has most widely been employed for display devices. As display mode applied for liquid crystal display, TN (twisted nematic) display mode, DS (dynamic scattering) display mode, guest-host display mode, and DAP (Deformation of Aligned Phases) display mode have been developed corresponding to electro-optic effects.
In recent years, coloring of liquid crystal displays has rapidly been advanced, and thin film transistor (TFT) display mode and super twisted nematic (STN) display mode are main streams in TN display mode as display mode. On the other hand, CRT which is a main stream of current television screen is expected to be replaced by liquid crystal displays sooner or later. In order to realize the replacement, liquid crystal displays must have display characteristics comparable to those of CRT.
In the research and development of liquid crystal displays, one""s energies have been devoted to the improvement of response speed, contrast, and viewing angle as important subject. Among them, response speed and contrast became such an extent as equal to those of CRT as a result of repeated improvements in TFT display mode. However, a wide viewing angle comparable to that of CRT has not yet been actualized, whereas some improvements such as an improvement in the orientational direction of liquid crystal molecules and the use of a phase difference plate have been made as to viewing angle.
Although it is an active matrix mode similar to that of TFT display mode, in-plane-switching (IPS) display mode which is characterized in that comb type electrodes are formed only one side of substrate is lately performed on the stage as a mode for actualizing a wide viewing angle (G. Baur, Freiburger Arbeistagung Flussigkristalle, Abstract No. 22 (1993) and M. Oh-e et al., ASIA DISPLAY ""95, 577 (1995)). When liquid crystalline compounds having a negative dielectric anisotropy value (xcex94xcex5) was used in IPS display mode, a dramatically wide viewing angle was obtained.
However, this IPS display mode has such a defect that response speed is considerably low compared with conventional TFT display mode or STN display mode. Then, liquid crystalline compounds having a negative and large xcex94xcex5 and a low viscosity have been required in IPS display mode.
Also, since active matrix driving mode is employed in IPS mode as described above, liquid crystalline compounds having a high voltage holding ratio (V. H. R.) are more preferable.
Various compounds having a negative dielectric anisotropy value are already known. In Laid-open Japanese Patent Publication No. Hei 2-4724 and Tokuhyo (Laid-open Japanese WO publication) No. Hei 2-503441, compounds having 2,3-difluoro-1,4-phenylene group in their partial structure are disclosed as liquid crystal compound having a negative xcex94xcex5.
It is considered that in the compounds having such partial structure, fluorine atoms substituted at positions 2 and 3 act so as to increase dipole moment in the direction of the minor axis of molecules to make dipole moment of the major axis smaller than the dipole moment in the direction of minor axis, and as the result, the compounds come to have a negative dielectric anisotropy value. However, compounds having such partial structure become slightly narrow in their temperature range exhibiting a liquid crystal phase compared with compounds in which hydrogen atoms of phenylene group are not replaced by fluorine atoms, their miscibility with other liquid crystalline compounds particularly at very low temperatures can hardly be said to be excellent, and sometimes such phenomena that smectic phase is developed and crystals are separated in liquid crystal compositions in a low temperature region are observed.
Compounds expressed by the following formula (a) are described in Tokuhyo No. Hei 2-503441: 
wherein R and Rxe2x80x2 represent an alkyl group and alkoxy group, respectively.
Whereas structural formula of the compounds is described in the publication mentioned above, physical properties and the likes necessary for judging the utility of the compounds as liquid crystalline compound are not described at all therein. Based on the consideration by the present inventors, whereas an improvement in miscibility by the compounds of the formula (a) described above compared with compounds having no 1,2-ethylene group can be surmised since the compounds of the formula (a) have 1,2-ethylene group as bonding group in skeleton structure, their effect can not be said to be sufficient.
An object of the present invention is to provide liquid crystalline compounds which are wide particularly in temperature range of liquid crystal phase, have a low viscosity, have a negative and large xcex94xcex5, and are improved in solubility at low temperatures; to provide liquid crystal compositions comprising the compound; and to provide liquid crystal display devices fabricated by using the liquid crystal composition, thereby to overcome the problems in conventional technologies described above.
Then, compounds expressed by the general formula (1) and simultaneously having butylene group or propylenoxy group, and 2,3-difluoro-1,4-phenylene group in the structure of compounds were diligently investigated by the present inventors. As the result of the investigation, it has been found out that the compounds are characterized in that they are wide in temperature range exhibiting a liquid crystal phase, are low in viscosity, and have a negative and large xcex94∈, as well as they are remarkably excellent in miscibility at low temperatures, leading to the accomplishment of the present invention.
That is, the present invention is summarized as follows:
[1] A liquid crystalline compound expressed by the general formula (1) 
xe2x80x83wherein R1 represents an alkyl group having 1 to 15 carbon atoms in which alkyl group, not-adjacent any methylene group may be replaced by oxygen atom or vinylene group, and any hydrogen atom in the alkyl group may be replaced by fluorine atom; ring A1, ring A2, and ring A3 independently represent trans-1,4-cyclohexylene group, trans-1,4-silacyclohexylene group, pyrimidine-2,5-diyl group, pyridine-2,5-diyl group, 1,3-dioxane-2,5-diyl group, tetrahydropyran-2,5-diyl group, 1,3-dithian-2,5-diyl group, or tetrahydrothiopyran-2,5-diyl group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by a halogen atom; X1, X2, and X3 independently represent xe2x80x94(CH2)4xe2x80x94, xe2x80x94(CH2)3Oxe2x80x94, xe2x80x94O(CH2)3xe2x80x94, or single bond; Y1 represents hydrogen atom or an alkyl group having 1 to 15 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group; m and n are independently 0 or 1; and any atom which constitutes this compound may be replaced by its isotope.
[2] The liquid crystalline compound recited in paragraph [1] above wherein ring A1 represents trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X1 represents xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; and either m and n are 0 in the general formula (1).
[3] The liquid crystalline compound recited in paragraph [1] above wherein ring A1and ring A2 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X1 represents xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; X2 represents single bond; and m is 1 and n is 0 in the general formula (1).
[4] The liquid crystalline compound recited in paragraph [1] above wherein ring A1 and ring A2 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X2 represents xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; X1 represents single bond; and m is 1 and n is 0 in the general formula (1).
[5] The liquid crystalline compound recited in paragraph [1] above wherein ring A1, ring A2, and ring A3 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X1 represent xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; either X2 and X3 represent single bond; and m is 1 and n is 1 in the general formula (1).
[6] The liquid crystalline compound recited in paragraph [1] above wherein ring A1, ring A2, and ring A3 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X2 represents xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; either X1 and X3 represent single bond; and m is 1 and n is 1 in the general formula (1).
[7] The liquid crystalline compound recited in paragraph [1] above wherein ring A1, ring A2, and ring A3 independently represent trans-1,4-cyclohexylene group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by fluorine atom; X3 represents xe2x80x94(CH2)4xe2x80x94 or xe2x80x94(CH2)3Oxe2x80x94; either X1 and X2 represent single bond; and m is 1 and n is 1 in the general formula (1).
[8] A liquid crystal composition comprising at least two components and comprising at least one liquid crystalline compound expressed by the general formula (1) 
xe2x80x83wherein R1 represents an alkyl group having 1 to 15 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group, and any hydrogen atom in the alkyl group may be replaced by fluorine atom; ring A1, ring A2, and ring A3 independently represent trans-1,4-cyclohexylene group, trans-1,4-silacyclohexylene group, pyrimidine-2,5-diyl group, pyridine-2,5-diyl group, 1,3-dioxane-2,5-diyl group, tetrahydropyran-2,5-diyl group, 1,3-dithian-2,5-diyl group, or tetrahydrothiopyran-2,5-diyl group, or 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by a halogen atom; X1, X2, and X3 independently represent xe2x80x94(CH2)4xe2x80x94, xe2x80x94(CH2)3Oxe2x80x94, xe2x80x94O(CH2)3xe2x80x94, or single bond; Y1 represents hydrogen atom or an alkyl group having 1 to 15 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group; m and n are independently 0 or 1; and any atom which constitutes this compound may be replaced by its isotope.
[9] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, and comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4) 
xe2x80x83wherein R2 represents an alkyl group having 1 to 10 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group; and any hydrogen atom in the alkyl group may be replaced by fluorine atom; Y2 represents fluorine atom, chlorine atom, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H, or xe2x80x94OCF2CFHCF3; L1 and L2 independently represent hydrogen atom or fluorine atom; Z1 and Z2 independently represent 1,2-ethylene group, vinylene group, 1,4-butylene group, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, or single bond; ring B represents trans-1,4-cyclohexylene group or 1,3-dioxane-2,5-diyl group, or 1,4-phenylene group in which hydrogen atom may be replaced by fluorine atom; ring C represents trans-1,4-cyclohexylene group, or 1,4-phenylene group in which hydrogen atom may be replaced by fluorine atom; and each atom which constitutes those compounds may be replaced by its isotope.
[10] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, and comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) 
xe2x80x83wherein R3 and R4 independently represent an alkyl group having 1 to 10 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group, and any hydrogen atom in the alkyl group may be replaced by fluorine atom; Y3 represents xe2x80x94CN or xe2x80x94Cxe2x89xa1Cxe2x80x94CN; ring D represents trans-1,4-cyclohexylene group, 1,4-phenylene group, pyrimidine-2,5-diyl group, or 1,3-dioxane-2,5-diyl group; ring E represents trans-1,4-cyclohexylene group or pyrimidine-2,5-diyl group, or 1,4-phenylene group in which hydrogen atom may be replaced by fluorine atom; ring F represents trans-1,4-cyclohexylene group or 1,4-phenylene group; Z3 represents 1,2-ethylene group, xe2x80x94COOxe2x80x94, or single bond; L3, L4, and L5 independently represent hydrogen atom or fluorine atom; a, b, and c are independently 0 or 1; and each atom which constitutes those compounds may be replaced by its isotope.
[11] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4), and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) 
xe2x80x83wherein R5 and R6 independently represent an alkyl group having 1 to 10 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group, and any hydrogen atom in the alkyl group may be replaced by fluorine atom; ring G, ring I, and ring J independently represent trans-1,4-cyclohexylene group or pyrimidine-2,5-diyl group, or 1,4-phenylene group in which one hydrogen atom may be replaced by fluorine atom; Z4 and Z5 independently represent 1,2-ethylene group, vinylene group, xe2x80x94COOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or single bond; and each atom which constitutes those compounds may be replaced by its isotope.
[12] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4), and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (10), (11), and (12) 
xe2x80x83wherein R7 and R8 independently represent an alkyl group having 1 to 10 carbon atoms in which alkyl group not-adjacent any methylene group may be replaced by oxygen atom or vinylene group, and any hydrogen atom in the alkyl group may be replaced by fluorine atom; ring K and ring M independently represent trans-1,4-cyclohexylene or 1,4-phenylene; L6 and L7 independently represent hydrogen atom or fluorine atom, but in no case simultaneously represent L6 and L7 hydrogen atom; Z6 and Z7 independently represent xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, or single bond; and each atom which constitutes those compounds may be replaced by its isotope.
[13] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (10), (11), and (12) described above.
[14] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above.
[15] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above.
[16] A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs [1] to [7] above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4) described above, comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) described above, and comprising, as a fourth component, at least one component selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above.
[17] A liquid crystal composition comprising at least one optically active compound in addition to the liquid crystal composition recited in any one of paragraphs [8] to [16] above.
[18] A liquid crystal display device fabricated by using the liquid crystal composition recited in any one of paragraphs [8] to [17] above.
Liquid crystalline compounds of the present invention expressed by the general formula (1) are two to four rings compounds having butylene group or propylenoxy group, and 2,3-difluorophenyl group at the same time in the molecular structure. As a matter of course, these liquid crystalline compounds are extremely stable physically and chemically under the environment in which liquid crystal display devices are used, and the compounds are characterized in that they are wide in temperature range exhibiting a liquid crystal phase, excellent in solubility in liquid crystal compositions even at low temperatures, and low in viscosity, and have a negative and large xcex94xcex5.
As described in the section of BACKGROUND ART, whereas compounds having 2,3-difluoro-1,4-phenylene group as a partial structure are already disclosed in patent publications, it is a fact discovered for the first time by the present inventors that the compounds simultaneously having 1,4-butylene group or propylenoxy group as bonding group and 2,3-difluoro-1,4-phenylene group exhibit the characteristic described above, and it is difficult to expect such fact from conventional technology.
In the compounds of the present invention, it is possible to optionally adjust desired physical properties by selecting a proper ring structure, bonding group, and lateral structure among molecule constituting elements. Accordingly, novel liquid crystal compositions and liquid crystal display devices having excellent characteristics, specifically
1) having a wide temperature range of liquid crystal phase,
2) being low in viscosity, and having a negative and large xcex94xcex5,
3) separating no crystals and developing no smectic phase even at very low temperatures,
4) being physically and chemically stable, and being possible to expand the temperature range of their usage, to drive at a low voltage, and to realize a high speed response and high contrast can be provided by using the compound of the present invention as component of liquid crystal compositions.
While any of the compounds of the present invention exhibits preferable physical properties, liquid crystal compositions having physical properties suitable for their use can be produced by using the compound which is expressed by the general formula (1) in which ring A1, ring A2, ring A3, X1, X2, X3, m, and n are properly selected.
That is, when compounds having a negative and large xcex94xcex5 are necessary, it is sufficient to suitably select 2,3-difluoro-1,4-phenylene group for any one of ring A1, ring A2, ring A3, and when compounds having a high optical anisotropy value are necessary, it is sufficient to select compounds in which any one of ring A1, ring A2, and ring A3 is 1,4-phenylene group, and every one of X1, X2, and X3 is single bond. When compounds having their temperature range of liquid crystal phase at high temperature side are necessary, it is sufficient to suitably select three rings or four rings compounds, and when compounds having their temperature range of liquid crystal phase at low temperature side are necessary, it is sufficient to suitably select two rings compounds, respectively.
Compounds in which hydrogen atom on 1,4-phenylene group is replaced by fluorine atom exhibit an excellent solubility at low temperatures.
Compounds expressed by one of the following general formulas (1-1) to (1-12) can be mentioned as particularly preferable ones among the compounds expressed by the general formula (1): 
wherein R1, ring A1, ring A2, ring A3, and Y1 have the same meaning as described above.
In the compounds described above, while R1 represents an alkoxy group, alkoxyalkyl group, alkenyl group, alkenyloxy group, alkenyloxyalkyl group, or alkyloxyalkenyl group having 1 to 15 carbon atoms, particularly preferable groups among them are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, methoxy, ethoxy, propoxy, butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, methoxyethyl, ethoxyethyl, propoxyethyl, methoxypropyl, ethoxypropyl, propoxypropyl, vinyl, 1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1-pentenyl, 2-pentenyl, 3-pentenyl, 4-pentenyl, 2-propenyloxy, 2-butenyloxy, 2-pentenyloxy, 4-pentynyloxy, methoxy-1-propenyl, methoxy-1-pentenyl, and methoxy-3-pentenyl.
In the compounds described above, while Y1 represents hydrogen atom, an alkyl group, alkoxy group, alkoxyalkyl group, alkenyl group, alkenyloxy group, alkenyloxyalkyl group, or alkyloxyalkenyl group having 1 to 15 carbon atoms, particularly preferable groups among them are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, methoxy, ethoxy, propoxy, butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, methoxyethyl, ethoxyethyl, propoxyethyl, methoxypropyl, ethoxypropyl, and propoxypropyl.
Liquid crystal compositions of the present invention are described below. Liquid crystal compositions of the present invention preferably comprise at least one compound expressed by the general formula (1) in the ratio of 0.1 to 99.9% by weight to develop excellent characteristics.
More preferably, the liquid crystal compositions provided by the present invention are completed by mixing compounds selected from the group consisting of the compounds expressed by one of the general formulas (2) to (12) depending on the purposes of the liquid crystal compositions in addition to the first component comprising at least one compound expressed by the general formula (1).
The present invention recited in the paragraphs [12] and [13] above are concerned with N type (having a negative xcex94xcex5) liquid crystal compositions. In the same way as P type (having a positive xcex94xcex5) liquid crystal compositions, N type liquid crystal compositions can be driven by various driving modes, for example, by IPS mode (In Plane Switching Mode). The present invention recited in paragraphs [9], [10], [11], [14], [15], and [16] are concerned with P type liquid crystal compositions. It is possible to control elastic constants of liquid crystal compositions and to improve the miscibility of the compositions at low temperatures by adding a N type liquid crystalline compound to P type liquid crystal compositions.
Following compounds can preferably be mentioned as ones used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (2) to (4): 
wherein R2 and Y2 have the same meaning as described above.
Compounds expressed by one of the general formulas (2) to (4) have a positive dielectric anisotropy value, are remarkably excellent in thermal stability and chemical stability, and are useful when liquid crystal compositions for TFT (AM-LCD) display mode of which a high reliability such as a particularly high voltage holding ratio or large specific resistivity is required are produced.
When the liquid crystal compositions for TFT display mode are produced, the compounds expressed by one of the general formulas (2) to (4) can be used in the range of 0.1 to 99.9% by weight based on the total amount of liquid crystal composition, and the amount is preferably 10 to 97% by weight and more desirably 40 to 95% by weight. Also, the compositions may further comprise the compound expressed by one of the general formulas (7) to (9) for the purpose of adjusting viscosity. Even when liquid crystal compositions for STN display mode or TN display mode are produced, the compound expressed by one of the general formulas (2) to (4) can be used. In this case, the amount of the compound to be used is preferably less than 50% by weight.
As the compound used in the liquid crystal compositions of the present invention expressed by the general formula (5) or (6), the following compounds can preferably be mentioned: 
wherein R3, R4, and Y3 have the same meaning as described above.
Compounds expressed by the general formula (5) or (6) have a positive and large dielectric anisotropy value, and are used particularly for the purpose of lowering threshold voltage of liquid crystal compositions. Also, they are used for the purpose of adjusting optical anisotropy value, and widening nematic range such as raising clearing point. Further, they are used even for the purpose of improving the steepness of V-T curve of liquid crystal compositions for STN display mode or TN display mode.
Compounds expressed by the general formula (5) or (6) are useful when liquid crystal compositions particularly for STN display mode or TN display mode are produced.
When the content of the compound expressed by the general formula (5) or (6) in liquid crystal compositions is increased, threshold voltage of liquid crystal compositions lowers but viscosity increases. Accordingly, it is advantageous to use the compound in a large amount since driving at a low voltage becomes possible, so far as viscosity of liquid crystal compositions satisfies required characteristics.
When liquid crystal compositions for STN display mode or TN display mode are produced, the amount of the compound expressed by the general formula (5) or (6) to be used is in the range of 0.1 to 99.9% by weight, preferably 10 to 97% by weight, and more desirably 40 to 95% by weight.
As the compounds used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (7) to (9), the following compounds can preferably be mentioned. 
wherein R5 and R6 have the same meaning as described above.
Compounds expressed by one of the general formulas (7) to (9) have a small absolute value of dielectric anisotropy, and are close to neutral. Compounds expressed by the general formula (7) are used principally for the purpose of adjusting viscosity and adjusting optical anisotropy value of liquid crystal compositions. Compounds expressed by the general formula (8) or (9) are used for the purpose of widening nematic range such as raising clearing point, or for the purpose of adjusting optical anisotropy value.
When the content of the compound expressed by one of the general formulas (7) to (9) in liquid crystal compositions is increased, threshold voltage of liquid crystal compositions rises but viscosity reduces. Accordingly, it is desirable to use the compound in a large amount so far as threshold voltage of liquid crystal compositions satisfies required characteristics. When liquid crystal compositions for TFT are produced, the amount of the compound expressed by one of the general formulas (7) to (9) to be used is preferably less than 40% by weight and more desirably less than 35% by weight. When liquid crystal compositions for STN display mode or TN display mode are produced, the amount is preferably less than 70% by weight and more desirably less than 60% by weight.
As the compounds used in the liquid crystal compositions of the present invention and expressed by one of the general formulas (10) to (12), the following compounds can preferably be mentioned: 
wherein R7 and R8 have the same meaning as described above.
Compounds expressed by one of the general formulas (10) to (12) have a negative dielectric anisotropy value. Compounds expressed by the general formula (10) are two rings compounds, and are used principally for the purpose of adjusting threshold voltage, adjusting viscosity, or adjusting optical anisotropy value. Compounds expressed by the general formula (11) are used for the purpose of widening nematic range such as raising clearing point, or for the purpose of adjusting optical anisotropy value. Compounds expressed by the general formula (12) are used for the purpose of widening nematic range as well as for the purpose of lowering threshold voltage and for the purpose of increasing optical anisotropy value.
Compounds expressed by one of the general formulas (10) to (12) are used principally for N type (having a negative dielectric anisotropy xcex94xcex5) liquid crystal compositions. When the amount of the compound to be used is increased, threshold voltage of liquid crystal compositions lowers but viscosity increases. Accordingly, it is desirable to use the compound in a small amount so far as threshold voltage of liquid crystal compositions is satisfied. However, since these compounds have an absolute value of dielectric anisotropy value of lower than 5, when the amount of the compound used is less than 40% by weight, driving at a low voltage sometimes becomes impossible.
The amount of the compound expressed by one of the general formulas (10) to (12) to be used in liquid crystal compositions is preferably more than 40% by weight when liquid crystal compositions for N type TFT are produced and the amount is more desirably 50 to 95% by weight.
Further, for the purpose of control the elastic constants of liquid crystal compositions and regulating voltage-transmittance curve (V-T curve), the compound expressed by one of the general formulas (10) to (12) is sometimes added to P type (having positive dielectric anisotropy xcex94xcex5) liquid crystal compositions. In such case, the amount of the compound expressed by one of the general formulas (10) to (12) to be used in liquid crystal compositions is preferably less than 30% by weight.
With the exception of such specific cases as liquid crystal compositions for OCB (optically Compensated Birefringence) mode and the likes, an optically active compound is usually added to the liquid crystal compositions of the present invention for the purpose of inducing helical structure of liquid crystal composition to adjust required twist angle and to prevent reverse twist. While any known optically active compounds used for such purposes can be added in the liquid crystal compositions of the present invention, the following optically active compounds can be mentioned as preferable examples: 
These optically active compounds are usually added to liquid crystal compositions of the present invention to adjust their pitch of twist. The twist pitch is preferably adjusted in the range of 40 to 200 xcexcm in the case of liquid crystal compositions for TFT or TN, and preferably adjusted in the range of 6 to 20 xcexcm in the case of liquid crystal compositions for STN. In the case for bistable TN mode, it is preferable to adjust the pitch in the range of 1.5 to 4 xcexcm. Further, two or more kind of optically active compounds may be added for the purpose of adjusting the dependency of the pitch length on temperature.
Liquid crystal compositions of the present invention can be produced by methods which are conventional by themselves. Generally, a method in which various components are dissolved one another at a high temperature has been adopted.
Further, the liquid crystal compositions of the present invention can be used as ones for guest-host (GH) mode by adding a dichroic dye such as merocyanine type, styryl type, azo type, azomethine type, azoxy type, quinophthalone type, anthraquinone type, and tetrazine type thereto. Alternatively, the liquid crystal compositions can be used as NCAP which is prepared by the microencapsulation of a nematic liquid crystal, or as liquid crystal compositions for polymer dispersed liquid crystal display devices (PDLCD) represented by polymer net work liquid crystal display devices (PNLCD) prepared by forming a polymer of three-dimensional reticulated structure in a liquid crystal. Still further, the liquid crystal compositions of the present invention can be used as ones for electrically controlled birefringence (ECB) mode or dynamic scattering (DS) mode.
[Methods for Producing Compounds]
Compounds of the present invention expressed by the general formula (1) can readily be produced by using ordinary chemical procedures of organic synthesis. For instance, the compounds can readily be synthesized by selecting proper known reactions described in reference books such as Organic Synthesis, Organic Reactions, and Shin-Jikken Kagaku Kouza (Course of New Chemical Experiment), and magazines, and using the reactions in combination.
When butylene group is introduced at the position of a bonding group (X1, X2, and X3), the compounds can be produced, for instance, by the following reaction paths.
In the following, MSG1 and MSG2 independently represent a mesogen (a residue of organic compounds); Hal represents Cl, Br, or I; ring A represents trans-1,4-cyclohexylene group, 1,4-phenylene group in which one or more hydrogen atoms on the six-membered ring may be replaced by a halogen atom, pyrimidine-2,5-diyl group, pyridine-2,5-diyl group, 1,3-dioxane-2,5-diyl group, tetrahydropyran-2,5-diyl group, 1,3-dithian-2,5-diyl group, or tetrahydrothiopyran-2,5-diyl group; and Y1 have the same meaning as described above.
That is, 2-(1,3-dioxane-2-yl)ethyltriphenylphosphonium halide (12) and aldehyde derivative (11) are subjected to the Wittig reaction in an ether type solvent such as tetrahydrofuran (hereinafter abbreviated to THF) and diethyl ether in the presence of a base such as sodium methylate, potassium-t-butoxide (t-BuOK), and butyl lithium to obtain compound (13). Subsequently, aldehyde derivative (14) can be obtained by subjecting compound (13) to hydrogen reduction in a mixed solvent of toluene/Solmix in the presence of a metal catalyst such as palladium/carbon and Raney nickel, and then reacting with a mineral acid such as hydrochloric acid and sulfuric acid, or an organic acid such as formic acid and p-toluenesulfonic acid.
Further, in the same way as that wherein compound (13) is obtained from compound (11), compound (16) can be obtained by subjecting compound (14) and compound (15) to the wittig reaction, and aldehyde derivative (17) can be produced by reacting it with the same acid as described above. Subsequently, derivative (19) having butylene group can be produced by reacting Grignard reagent (18) with compound (17) to conduct Grignard reaction, reacting it with the same acid as described above to dehydrate, and further subjecting to hydrogen reduction by using the same metal catalyst as described above. 
When propylenoxy group having ether bond is introduced at the position of a bonding group (X1, X2, and X3), the compounds can be produced, for instance, by the following reaction paths.
Aldehyde derivative (14) is reacted with lithium aluminum hydride in a solvent such as toluene, THF, and diethyl ether to reduce thereby to obtain alcohol derivative (20). This alcohol derivative (20) is reacted with hydrobromic acid to produce compound (21). Compound (23) having ether bond can be produced by reacting compound (21) with compound (22) in the presence of sodium hydride. 
When 2,3-difluoro-1,4-phenylene group is introduced to a ring structure portion, the compounds can be produced, for instance, by the following reaction paths.
a) The case wherein the introduction portion is located at position 4 relative to MSG1 of benzene derivative:
Compound (25) can be obtained by reacting difluorobenzene derivative (24) with n-butyl lithium or sec-butyl lithium in an ether type solvent such as THF and diethyl ether, reacting with zinc chloride, and then reacting with 2,3-difluoro-1-bromobenzene in the presence of a metal catalyst of palladium (0).
b) The case wherein it is introduced in cyclohexanone derivative having MSG1 at position 4:
Compound (28) can be produced by reacting compound (26) with Grignard reagent (27) to conduct the Grignard reaction, dehydrating by the same procedure as described above, and then subjecting to hydrogen reduction. 
Compounds in which ring A1, ring A2, and ring A3 are silacyclohexane rings can be produced according to the method disclosed in Laid-open Japanese Patent Publication No. Hei 7-70148, Laid-open Japanese Patent Publication No. 7-112990, and Laid-open Japanese Patent Publication Nol Hei 7-149770.
Compounds of the present invention expressed by the general formula (1) can be produced by selecting and using proper reactions described above.
Any of the liquid crystalline compounds of the present invention expressed by the general formula (1) thus obtained has such characteristics that the temperature range in which the compound exhibits a liquid crystal phase is wide, viscosity is low, and xcex94xcex5 is negative and large, and the compound is readily mixed with other various liquid crystal materials even at low temperatures. Accordingly, the compound is remarkably excellent as constituent of nematic liquid crystal compositions suitable for TFT type display mode and IPS mode.